Ball valves are employed in a variety of applications in hydraulic systems. For example, ball valves are used as shut-off valves to separate branches of hydraulic systems, as safety valves during maintenance of hydraulic systems, and as devices for controlling flow direction.
The general trend in the fluid power industry is toward higher system pressures. This has, in turn, exacerbated the problems in operating ball valves. It has long been recognized that it is extremely difficult, and in some cases almost impossible, to operate a ball valve under pressure, especially to open one. Clearly, this situation is intolerable, especially in an emergency situation.
Ball valves belong to the group of so-called "non-restrictive flow" valves. The closing element of the valve--the ball--generally floats (i.e., is not rigidly mounted) with respect to the valve body, and rotates within annular valve seats on either side of the ball. Typically, the valve seats are plastic. The ball is exposed to fluid pressure over the area of the full (non-restricted) valve passage. Because the ball presents a substantial area to the pressurized fluid, the fluid forces the ball against the valve seat with great force. Since force is the product of pressure multiplied by area, the higher the pressure, and the higher the area, the greater the force. If the valve is left in the closed position for a sufficient period of time (i.e., three hours or longer), the friction between the valve seat and the ball becomes enormous. This is due primarily to two factors. First, the normally-present lubricating film on the contact surfaces of the ball and valve seats is squeezed out, and hence dry friction is substituted for wet friction. It should be apparent that dry friction will be much greater than wet friction. Second, the plastic material of the seat deforms under pressure, and will be forced into and fill the microscopic voids inevitably present on the surface of the ball. As a result of these two phenomena, the resistance to opening can be so high that the ball is virtually impossible to rotate by hand or even with a wrench.
Prior attempts to solve this problem have focused on reducing the diameter of the flow passage through the ball, reducing the system pressure, or using a parallel pressure equalization line between upstream and downstream sides of the valve. These techniques are clearly unsatisfactory. The first two compromise the main advantages of ball valves: unrestricted flow and the ability to withstand high pressures. The third method requires the installation of additional piping, fittings and valves outside of the valve body, which makes that method very expensive and not always feasible, such as in installations where space is limited.
There is therefore a need to provide a ball valve which permits unrestricted flow and which operates at high pressures, and which overcomes the difficulties associated with prior art ball valves. The present invention fills that need.